Two immunoglobulin tandem proteins with a linking β-strand reveal unexpected differences in cooperativity and folding pathways.
Bottom Line: The structures of the tandem immunoglobulin (Ig) domain pairs A164-A165 and A168-A169, from the A-band of the giant muscle protein titin, reveal that they form tightly associated domain arrangements, connected by a continuous β-strand.We elucidate the folding and unfolding pathways of both tandem pairs and show that cooperativity in A164-A165 is a manifestation of the relative refolding and unfolding rate constants of each individual domain.We infer that the differences between the two tandem pairs result from a different pattern of interactions at the domain/domain interface.
Affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.Show MeSH
Mentions: The A-band of titin is made up of Ig and FNIII domains, which are mostly arranged in repeating patterns.17,18 A conformational characterization of three FNIII tandems from the A-band shows that these contrast markedly with Ig tandems from the elastic I-band: the FNIII interfaces were found to be conformationally well defined, exhibit limited dynamics, and are largely conserved.19 The Ig I-set domains A164, A165, A168, and A169 are located in the A-band segment of titin, N-terminal to the titin kinase domain and M-band.20,21 The linker between the two domains is one residue shorter than that between Ig domains in the I-band of titin. Importantly, in the tandem Ig pairs A164–A165 and A168–A169, the C-terminal G-strands of A164 and A168 are connected via a continuous β-strand with the N-terminal A-strands of A165 and A169, respectively (Fig. 1a). Here, we investigate the hypothesis that a continuous β-strand can impart cooperative folding between adjacent Ig domains, as a continuous α-helix does in spectrin R1617.22
Affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.